Question

我的Android项目中有两项活动。一个是MainActivity，另一个是AddContributor。我在MainActivity中有一个FloatingActionButton，我想用该按钮切换到AddContributor。我已经在fab的onClickListener中实现了我的Intent，并希望从onClickListener启动AddContributor。当我单击Button时，我得到一个NullPointerException，如下所示：

java.lang.RuntimeException: Unable to instantiate activityComponentInfo{de.money.splitthebill/de.money.splitthebill.AddContributor}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object     reference

MainActivity的代码在这里：

     package de.money.splitthebill;

import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;

/**
 * In diesem Bild soll die Liste der Teilnehmer angezeigt werden.
 * Der fab fuegt Teilnehmer zur Rechnung hinzu.
 */
public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);


        final FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);

        //Ändert das Symbol vom FAB

        fab.setImageResource(R.drawable.ic_action);

        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
               startActivity(new Intent(MainActivity.this, AddContributor.class)); // Für die Transition http://www.androidhive.info/2011/08/how-to-switch-between-activities-in-android/, für
                                                                                    //startActivity: http://stackoverflow.com/questions/30179914/start-a-new-activity-onclicklistener-android
            }
        });
    }



    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

AddContributor的代码在这里：

package de.money.splitthebill;

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

/**
 * Created by Timbo on 09/12/15.
 * Die Klasse beschreibt die Funktionalitaet der add_contributor.xml
 */
public class AddContributor extends AppCompatActivity{


    final EditText nameText = (EditText) findViewById(R.id.nameText);
    final EditText snackText = (EditText) findViewById(R.id.snackText);
    final EditText betragZahl = (EditText) findViewById(R.id.betragZahl);
    final Button fertigButton = (Button) findViewById(R.id.fertigButton);

    Intent mainAct = getIntent();


    public String _snackText = "";
    public String  _nameText = "";
    private String _betragZahl = "";
    public int _betrag = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.add_contributor);

        fertigButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View view) {
                Intent mainAct = new Intent(view.getContext(), MainActivity.class);
                startActivity(mainAct);

            }
        });
    }

    /**
     * Nimmt den Inhalt der EditText's und schreibt sie in Strings
     * Nur der Betrag wird in einen int gespeichert
     */

    private void getContent()
    {
        _snackText = snackText.getText().toString();
        _nameText = nameText.getText().toString();
        _betragZahl = betragZahl.getText().toString();
        parseBetrag(_betragZahl);
    }

    /**
     * Parst den String aus _betragZahl in den int _betrag
     * @param betrag Der String der aus getContent() uebergeben wird
     * @return ein Integer zur weiteren Verwendung der den Betrag ohne Komma in Cent enhaelt
     */

    private int parseBetrag(String betrag)
    {
        int zaehler = 0;
        for (int i = 0; i < _betragZahl.length(); ++i)
        {
            if (betrag.charAt(i) == ',')
            {
                String subEins = betrag.substring (0, i -1);
                String subZwei = betrag.substring(i +1, betrag.length() -1);
                _betragZahl = subEins + subZwei;
            }
        }
         return (_betrag =Integer.parseInt(_betragZahl));
    }



}

MainActivity的.xml文件位于：

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context="de.money.splitthebill.MainActivity">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>

    <include layout="@layout/content_main" />

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        android:onClick="onClick"
        android:src="@drawable/ic_action" />

</android.support.design.widget.CoordinatorLayout>

AddContributor .xml在这里：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent">

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="textPersonName"
        android:text="Name"
        android:ems="10"
        android:id="@+id/nameText"
        android:layout_marginTop="53dp"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginLeft="41dp"
        android:layout_marginStart="41dp" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/snackText"
        android:text="Snack"
        android:layout_marginTop="48dp"
        android:layout_below="@+id/nameText"
        android:layout_alignLeft="@+id/nameText"
        android:layout_alignStart="@+id/nameText"
        android:layout_alignRight="@+id/nameText"
        android:layout_alignEnd="@+id/nameText" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="numberDecimal"
        android:ems="10"
        android:id="@+id/betragZahl"
        android:text="Betrag"
        android:layout_centerVertical="true"
        android:layout_alignLeft="@+id/snackText"
        android:layout_alignStart="@+id/snackText" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Geld geben  "
        android:id="@+id/geldButton"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginBottom="45dp" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:text="Fertig"
        android:id="@+id/fertigButton"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />

</RelativeLayout>

最后是AndroidManifest.xml：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="de.money.splitthebill">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>


        <activity
            android:name=".AddContributor"
            android:label="@string/app_name">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="de.money.splitthebill.MainActivity" />
            </activity>

    </application>

</manifest>

希望有人能够指出我所犯的错误。我确信这是一个非常简单和愚蠢的错误，但我现在无法找到它超过一个星期。谢谢你的帮助！

Answer 1

将setContentView之后的代码放在第二个活动中：

final EditText nameText = (EditText) findViewById(R.id.nameText);
    final EditText snackText = (EditText) findViewById(R.id.snackText);
    final EditText betragZahl = (EditText) findViewById(R.id.betragZahl);
    final Button fertigButton = (Button) findViewById(R.id.fertigButton);

如果内容已知，则可以指定身份证。

在Android

1 个答案: